2 files changed, 79 insertions, 0 deletions

diff --git a/crates/ra_hir_def/src/nameres.rs b/crates/ra_hir_def/src/nameres.rs
index 5893708e8..11ba8a777 100644
--- a/crates/ra_hir_def/src/nameres.rs
+++ b/crates/ra_hir_def/src/nameres.rs
@@ -1,3 +1,5 @@
 //! FIXME: write short doc here
 
+// FIXME: review privacy of submodules
 pub mod raw;
+pub mod mod_resolution;
diff --git a/crates/ra_hir_def/src/nameres/mod_resolution.rs b/crates/ra_hir_def/src/nameres/mod_resolution.rs
new file mode 100644
index 000000000..7d7e2779a
--- /dev/null
+++ b/crates/ra_hir_def/src/nameres/mod_resolution.rs
@@ -0,0 +1,77 @@
+//! This module resolves `mod foo;` declaration to file.
+use hir_expand::name::Name;
+use ra_db::FileId;
+use ra_syntax::SmolStr;
+use relative_path::RelativePathBuf;
+
+use crate::{db::DefDatabase2, HirFileId};
+
+#[derive(Clone, Debug)]
+pub struct ModDir {
+    /// `.` for `mod.rs`, `lib.rs`
+    /// `./foo` for `foo.rs`
+    /// `./foo/bar` for `mod bar { mod x; }` nested in `foo.rs`
+    path: RelativePathBuf,
+    /// inside `./foo.rs`, mods with `#[path]` should *not* be relative to `./foo/`
+    root_non_dir_owner: bool,
+}
+
+impl ModDir {
+    pub fn root() -> ModDir {
+        ModDir { path: RelativePathBuf::default(), root_non_dir_owner: false }
+    }
+
+    pub fn descend_into_definition(&self, name: &Name, attr_path: Option<&SmolStr>) -> ModDir {
+        let mut path = self.path.clone();
+        match attr_to_path(attr_path) {
+            None => path.push(&name.to_string()),
+            Some(attr_path) => {
+                if self.root_non_dir_owner {
+                    assert!(path.pop());
+                }
+                path.push(attr_path);
+            }
+        }
+        ModDir { path, root_non_dir_owner: false }
+    }
+
+    pub fn resolve_declaration(
+        &self,
+        db: &impl DefDatabase2,
+        file_id: HirFileId,
+        name: &Name,
+        attr_path: Option<&SmolStr>,
+    ) -> Result<(FileId, ModDir), RelativePathBuf> {
+        let file_id = file_id.original_file(db);
+
+        let mut candidate_files = Vec::new();
+        match attr_to_path(attr_path) {
+            Some(attr_path) => {
+                let base =
+                    if self.root_non_dir_owner { self.path.parent().unwrap() } else { &self.path };
+                candidate_files.push(base.join(attr_path))
+            }
+            None => {
+                candidate_files.push(self.path.join(&format!("{}.rs", name)));
+                candidate_files.push(self.path.join(&format!("{}/mod.rs", name)));
+            }
+        };
+
+        for candidate in candidate_files.iter() {
+            if let Some(file_id) = db.resolve_relative_path(file_id, candidate) {
+                let mut root_non_dir_owner = false;
+                let mut mod_path = RelativePathBuf::new();
+                if !(candidate.ends_with("mod.rs") || attr_path.is_some()) {
+                    root_non_dir_owner = true;
+                    mod_path.push(&name.to_string());
+                }
+                return Ok((file_id, ModDir { path: mod_path, root_non_dir_owner }));
+            }
+        }
+        Err(candidate_files.remove(0))
+    }
+}
+
+fn attr_to_path(attr: Option<&SmolStr>) -> Option<RelativePathBuf> {
+    attr.and_then(|it| RelativePathBuf::from_path(&it.replace("\\", "/")).ok())
+}